Broadcast and Multicast are methods for transmitting data-grams from a single source to several destinations, i.e. as point-to-multipoint transmission.
As mentioned in the technical specification 3GPP TS 22.146 V5.0.0 (2001-10): “Multimedia Broadcast/Multicast Service; Stage 1 (Release 5)”, currently two such services are specified for cellular networks, the cell broadcast service (CBS) and the internet protocol (IP) multicast service.
The cell broadcast service enables a transmission of low bit-rate data to all subscribers in a set of given radio cells of a cellular network over a shared broadcast channel.
The transmission of content data for IP multicast services, provided e.g. by an internet service provider, to user terminals via a radio interface can currently be supported by GPRS (general packet radio system) based cellular networks. It is required, however, that a point-to-point PDP (packet data protocol) context is set up between every user terminal and a network element of the GPRS network, i.e. the GGSN (Gateway GPRS support node), and every user terminal receiving a IP multicast transmission occupies a separate radio channel. Several radio channels thus deliver the same content in a point-to-point way through a GPRS system.
For UMTS, 3GPP (3rd generation partnership project) is developing new services for multicast and broadcast in UMTS. In the above mentioned technical specification 22.146, 3GPP provides for example a stage one description of broadcast and multicast services for UMTS. One desirable feature for new multicast services is for example to enable the transmission of an IP-multicast service content via a radio access network of a UMTS system to several user terminals with a shared radio channel. For both, new broadcast and new multicast services, a procedure for activating a broadcast or multicast session on the radio interface still has to be provided.
From an IP multicast content source point of view, the handling of a multicast transmission by a radio access network should be similar to IP-multicast, in order to avoid a conflict with the IETF IP-Multicasting standards.
In the RFC (Request for Comments) 2236: “Internet Group Management Protocol, Version 2”, of November 1997, it is specified for example that multicast routers use IGMP (Internet Group Management Protocol) to learn which groups have members on each of their attached physical networks. It is requested that a multicast router keeps a list of multicast group memberships for each attached network, and a timer for each membership. The expression ‘Multicast group memberships’ means the presence of at least one member of a multicast group on a given attached network, not a list of all of the members.